(532a) Gelation Dynamics during Photocrosslinking of Polymer Nanocomposite Hydrogels

Hydrogels have found widespread utility in applications ranging from tissue engineering to agriculture; however, they often exhibit relatively poor mechanical properties and limited functionality in the absence of nanomaterial fillers. Embedding nanocapsules into hydrogels presents an engineering opportunity to simultaneously improve the mechanical properties and incorporate functionality into hydrogels. The gelation kinetics of network-forming polymer solutions in the absence and presence of silica-coated nanocapsules were investigated using in situ dynamic rheology. Network-forming polymers comprised either 4-arm or 8-arm star polyethylene glycol (PEG) with terminal anthracene groups, which form dimers upon irradiation with ultraviolet (UV) light. Upon UV exposure, 4- and 8-arm PEG-anthracene solutions exhibited rapid gel formation as indicated by crossovers from liquid-like to solid-like behavior during in situ small-amplitude oscillatory shear rheology. Nanocomposite solutions were formulated to maintain fixed effective polymer concentration across a range of nanocapsule concentrations by accounting for the volume that nanocapsules exclude in solution. Adding nanocapsules to PEG-anthracene solutions resulted in faster gelation and stiffer hydrogels than nanocapsule-free PEG-anthracene solutions with equivalent effective polymer concentrations. Faster gelation of the nanocomposite hydrogels was attributed to a decrease in length scale necessary for network percolation, which was dependent on the number of star polymer arms. The final elastic modulus of nanocomposite hydrogels increased with nanocapsule volume fraction, signifying synergistic mechanical reinforcement by nanocapsules. Overall, these findings quantify the impact of nanocapsule addition on the gelation kinetics and mechanical properties of polymer nanocomposite hydrogels, which are promising materials for applications in optoelectronics, biotechnology, and additive manufacturing.